This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. Mercury is one of the most toxic elements in nature, producing kidney failure and neurological disorders in humans (1). Inorganic mercury results toxic because it reacts with the sulfur atoms in proteins interfering with their physiological function. Our aim is to understand at molecular level how bacteria resist mercury, in particular, how the protein MerT transports mercury. A better understanding of bacterial mercury resistance could lead to the implementation of improved bioremediation schemes to decontaminate waters and soils of mercury pollution. Bacteria avoid mercury toxicity by transporting the mercury found in the periplasm to the cytoplasm where the toxic Hg2+ ion is reduced to the less toxic and volatile Hg0 form (2). Three proteins in the bacterial mer operon are directly involved in the transport mechanism: a periplasmic mercury binding protein (MerP), a transporter (MerT) and a mercury reductase (MerA) (3). Cysteine residues in the three proteins are known to be involved in the transport mechanism, but a defined pathway for the mercuric ion is yet to be determined. Our specific goal is to understand at an atomic level how periplasmic mercury is transported across the membrane by MerT to the cytoplasm.